Showing posts with label dysbiosis. Show all posts
Showing posts with label dysbiosis. Show all posts

Saturday, November 17, 2012

Amelioration of Colitis by Parasites - or "An Elliott & Weinstock Special"

Common parasites such as Blastocystis and Dientamoeba fragilis are often incriminated of causing chronic or intermittent diarrhoea or other intestinal symptoms despite the absence of compelling evidence. What most of us probably fail to realise is that parasites may actually prevent and ameliorate intestinal illness, including inflammatory bowel disease, other types of colitis, and other types of autoimmune diseases.

Inflammatory bowel disease (IBD) includes the two most common manifestations ulcerative colitis and Crohn’s Disease and affects more than 2 million people in North America and Europe. They are chronic inflammatory conditions of the gut that usually begin when people are in the second to third decade of life. Although the causes of these inflammatory diseases remain unknown, they are assumed to result from inappropriately aggressive mucosal (i.e. related to our intestinal lining) immune responses to elements or substances in our intestine. IBD is treated with immuno-suppresive drugs.

IBD has emerged primarily in the Western world along with a significant reduction in cases of intestinal helminthiasis due to clean food and water, improved hygiene and sanitation, and the development and use of antibiotics. In Denmark, helminthic infections due to previously common parasitic worms such as Ascaris (roundworm) are now at the point of being almost extinct in the indigenous population.

The hygiene hypothesis proposes that a causal link exists between the adoption of modern hygiene and the increase in the prevalence of immune dysfunctions. The extent of perinatal maturation of the immune system may play a crucial role in terms of our likelihood of developing allergic and autoimmune diseases later in life. The maturation process includes establishment of tolerance to food and harmless microorganisms, but also defence mechanisms against pathogens. If our environment is "too clean", we may fail to give our immune system the best possible opportunity to mature and differentiate appropriately. A robust immune response will protect us from recurrent infections, but if misdirected, it can cause disease.

Part of our immune system is the "adaptive immune system" -  or our "immunologic memory" - made up by cells such as lymphocytes (T- and B-cells), macrophages, dendritic cells, etc. plus antibodies and hormone-like substances (eg. cytokines) that are secreted to activate/inactivate or up- and down-regulate these cells. Our immune systems has to be able to recognise a plethora of foreign material such as bacteria, viruses and parasites, and to distinguish "self" from "non-self". IBD may be caused by mal-functions in our own immune system, and so may a lot of other diseases, diseases that we call "autoimmune diseases", and which include coeliac disease, multiple sclerosis, type 1 diabetes, and rheumatoid arthritis.

10,000 years ago, humans were infected by a variety of species of worms that are common in some parts of the world even today and hence humans and parasites have co-evolved over thousands of years. Importantly, most wild animals in their natural habitat are carriers of many types of parasites. A "clever" parasite does little harm to its host. Parasites have developed mechanisms that enable them to survive in their hosts, and also, the human immune system has developed a way to adapt to these common intruders.

Egg of Trichuris trichiura. Courtesy of Dr Marianne Lebbad.
How can one explain the amelioration of symptoms due to colitis by the presence of intestinal nematodes? Helminths appear to induce immune host regulatory cells that suppress inflammation, and helminth infections are strong inducers of immune regulatory circuits. The immune system changes in response to helminth colonisation and factors secreted by helminths that can influence immune cell function. It is likely that several immune-regulatory mechanisms are exploited by individual helminths. Otherwise, a helminth could not reliably evade our immune system to reproduce.

A new study has produced data that suggest that treatment of macaques suffering from chronic diarrhoea with eggs of the whipworm Trichuris suis can alleviate symptoms and modulate both the intestinal microbiota and immunoregulatory pathways. Trichuris suis is the whipworm of the pig, and contrary to Trichuris trichiura (image), T. suis appears not to be able to produce disease in primate hosts (including humans). When T. suis ova (TSO) are administered to humans, transient shedding of ova in faeces may be seen after a few weeks, but the individual remains asymptomatic.
Gene expression profiling of colonic biopsies from the macaques treated with TSO revealed up-regulation of genes typically involved in the so-called Th1-type immuno-response prior to TSO challenge, while induction of the Th2-type response followed after the TSO challenge; the Th2-type response resulted in mucosal repair, probably by increasing mucus production and turnover of epithelial cells, which again led to a reduction of bacterial attachment to the gut lining and a restoration of microbial diversity.

Briefly, a Th1-type response is generally a pro-inflammatory response that, among many other things, is responsible for microbicidal actions and perpetuating autoimmune responses. Excessive pro-inflammatory responses can lead to uncontrolled tissue damage, so there needs to be a mechanism to counteract this. The Th2­-type response includes the secretion of the anti-inflammatory cytokines, co-responsible for a general anti­-inflammatory response. In excess, Th2-type responses will counteract the Th1-mediated microbicidal action. The optimal scenario would therefore seem to be that humans should produce a well balanced Th1- and Th2-type response, suited to the immune challenge.
On top of the immunoregulatory impact, there is emerging evidence that helminths promote the growth and expansion of groups of bacteria that are beneficial or "probiotic" to the host. In the study of the macaques, the TSO induced a change in the intestinal microbiota.

While variation in160 genes in the human genome or more have been associated with increased risk of developing IBD, no specific gene variant that is sufficient or required for dysregulated mucosal inflammation as occurs in Crohn's disease or ulcerative colitis has been identified so far. There is a field of thought now saying that - over thousands of years - the human gut flora, including helminths, drove the development of variations in genes orchestrating various immune response pathways, and such genetic variations selected to operate under the influence of helminth infection could cause disease when operating without that influence.

So, the take home message here is that infestation by intestinal parasites may be a double-edged sword: While on one hand they may cause symptoms, they may on the other hand prevent us from developing inflammatory bowel disease and other autoimmune or allergic manifestations. Hence, helminths, although parasites, may contribute something in return to their hosts, and the loss of helminths removes a natural governor that helped to prevent disease due to immune regulation. Of course, more trials are needed before "helminth therapy" can actually be standardised, commercialised and used in the prophylaxis and treatment of IBD and gut allergic conditions. Once a good mechanistic understanding of how helminths alter immunity is available, it may even be possible to apply identified factors individually or in combination to treat disease.

As always, things are much more complex than presented here, but this post gives an impression of some of the fields of thought. Not all autoimmune diseases are driven by excessive Th1-type responses; some types of asthma may be driven by Th2-type response, but even here, helminths may favourably modulate immunoregulatory pathways.

Obviously, it would be interesting to explore how other parasitic infections impact on our immune system and gut flora. Interestingly, one helminth species appears to have "survived" in our "sterile" environment, - the pinworm (Enterobius)... and as pointed out in one of my recent blog posts (go here), many of us are definitely exposed to parasites that persist in our intestines for months, maybe years. What's their role in all of this?

Further reading:

Dirtying Up Our Diets - go here

Parasitic Worm Eggs Ease Intestinal Ills By Changing Gut Microbiota - go here.

Jostins L, et al. Host-microbe interactions have shaped the genetic architecture of inflammatory bowel disease. Nature, 491 (7422), 119-24 PMID: 23128233

Berger, A. (2000). Science commentary: Th1 and Th2 responses: what are they? BMJ, 321 (7258), 424-424 DOI: 10.1136/bmj.321.7258.424
Elliott, D., & Weinstock, J. (2012). Where are we on worms? Current Opinion in Gastroenterology, 28 (6), 551-556 DOI: 10.1097/MOG.0b013e3283572f73
Elliott, D., & Weinstock, J. (2012). Helminth-host immunological interactions: prevention and control of immune-mediated diseases Annals of the New York Academy of Sciences, 1247 (1), 83-96 DOI: 10.1111/j.1749-6632.2011.06292.x
Weinstock, J. (2012). Autoimmunity: The worm returns Nature, 491 (7423), 183-185 DOI: 10.1038/491183a

Elliott DE, Summers RW, & Weinstock JV (2007). Helminths as governors of immune-mediated inflammation. International journal for parasitology, 37 (5), 457-64 PMID: 17313951

Broadhurst, MJ., et al.Therapeutic helminth infection of macaques with idiopathic chronic diarrhoea alters the inflammatory signature and mucosal microbiota of the colon PLoS Pathogens (PLoS Pathog 8(11): e1003000. doi:10.1371/journal.ppat.1003000).

Friday, June 22, 2012

More Bits And Pieces On The Microbiome... Or Maybe Mycobiome...

I promised to include some more stuff from some of the many recent publications in Science and Science Translational Medicine on the intestinal microbiome and its potential role in health and disease, and I've chosen two papers that could have broad public interest; for those who need an introduction to the microbiome, please go here (Wikipedia entry).

Because the microbiome has been more or less exclusively synonymous with the "bacteriome" it's very refreshing to discover a paper on fungal diversity in the gut. Like Blastocystis, and other single-celled parasites, intestinal fungi are also micro-eukaryotes, and we are continuously searching for the role of micro-eukaryotes in health and disease.

In general, very little is known about fungi in the intestine, and most clinicians, even mycologists, hardly bother about the fungi that may be present in our intestine, - I think I can say that without offending anyone! Maybe one of the most interesting things in a clinical respect is the fact that antibodies against the yeast Saccharomyces cerevisiae (see below) is a common finding in patients with Crohn's Disease, but relatively uncommon in patients with ulcerative colitis and healthy individuals.

Now, Iliev et al. (2012) start out by confirming the fact that fungi are indeed common commensals and thus a part of our normal intestinal flora. They then showed that colitis chemically induced in mice led to circulating antibodies against S. cerevisiae, which suggested that fungal antigens commonly found in the gut might be responsible for the induction of these antifungal antibodies during colitis.
The innate immune receptor Dectin-1 appears to have a key role in fungal recognition and combating. Therefore the authors wanted to further explore the role of this receptor by studying mice with and without Dectin-1. They found that Dectin-1 deficiency led to increased susceptibility to chemically induced colitis, including weight loss, tissue destruction and cell infiltration by inflammatory cells, etc. Moreoever, evidence was found of fungal invasion of inflamed tissue in the Dectin-1 knockout mice and taken together, their data suggest that Dectin-1 deficiency leads to altered immunity to commensal gut fungi.
To cut a long story short, results from these experiments in mice led the investigators to search for mutations in CLEC1A (the human Dectin-1 gene) in patients with ulcerative colitis, and they found that mutations were significantly more common in patients with severe ulcerative colitis (patients requiring colectomy) than in those with a less aggressive disease progression. This suggests that not only bacteria but also intestinal fungi interact with the intestinal immune system and may thereby influence health and disease. If this can be confirmed by others, this is an example of how biomarkers can predict the disposition towards/progression of disease and the results may have profound consequences for diagnostic strategies (e.g. screening for mutations in the Dectin-1 gene) and therapeutic management of patients with severe ulcerative colitis. Maybe it would have been interesting to know about such mutations in patients with Crohn's Disease as well...

Next, the investigators took to identifying what types of fungi were actually present in the colon of these mice. What may be a little bit controversial is the fact that the authors - by amplification and deep sequencing of  ITS1-2 (genetic marker commonly used to identify and taxonomically group fungi) - appear to have found not only species representing a staggering 50 well-annotated fungal genera in the mouse microbiome, but an additional 100 "novel and/or un-annotated fungi" as well - this does sound like a lot, but somehow the reader is calmed down a bit, when the authors later tell us that 97.3% of all fungi detected in the mouse faeces belonged to only 10 species, with 65.2% of the fungal sequences belonging to Candida tropicalis. So, whether the 100 novel fungi are indeed fungi colonising the intestinal tract is unknown, but they may very well represent fungi "on transit", so to say, acquired from food, drink or environment maybe... we know that fungi are ubiquitous - we inhale fungi every day for instance, and when deep sequencing is applied, it may be possible to trace even fungi only present in very small quantities; also ITS-2 analysis does not tell us whether the sequences are from "intact/live" (i.e. colonising) fungi or from degraded fungi (i.e. ingested); a classic example is Saccharomyces cerevisiae (Brewer's or Baker's yeast), which we may often acquire from food and drink, but which may also colonise (settle and proliferate) our intestines. Contamination of the faecal samples from fungi present in the environment and during processing is also a possibility (one of the reasons why PCR-based diagnostics for fungal infections is a tricky task...). Well so, all of these new species/genera may not necessarily represent the "mouse mycobiome". However, the authors found only few of the fungi in the food that was fed to the mice, so this still may remain a bit of a mystery... it would have been interesting to know whether the fungi detected were yeasts or molds, for instance, and very little information can be extracted from the supplementary material (phylogenetic analysis) accompanying the paper. Anyway, it's all very stimulating and further studies will assist in exploring fungal diversity and, hopefully, the diversity of micro-eukaryotes in general.

Saccharomyces cerevisiae is used in food and drink, but may also colonise our guts.

The next paper is one of many recent papers heralding the implementation of microbiome-based therapies in future personalised and precision medicine, possibly relevant to diseases such as inflammatory bowel disease, obesity and diabetes. Microbiome manipulation, so to say, is key to this concept and includes controlled diet, pre- and probiotic interventions, bariatric surgery (e.g. gastric bypass), faecal transplants (see my recent blog post on feacal bacteriotherapy), helminth therapy (yes!) or ecological engineering. Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host, and these may be known to many as lactobacilli or bifidobacteria (or simply "yoghurt"!) that protect us against harmful bacteria by inhibiting their growth and by helping reduce cholesterol levels, synthesise vitamins and sustain immune responses. Prebiotics are non-digestible dietary sugar molecules (oligosaccharides) that can enhance the activity of for instance lactobacilli and bifidobacteria. While the potential benefits of pre- and probitics have been known for many years, it is only with current available technology that we are starting to get a mechanistic understanding of their impact on our bodies.

The article picks up on host-gut microbiota metabolic interactions and the so-called "host-microbe metabolic axes", which include pathways and interactions responsible for gut permeability, formation of blood vessels (angiogenesis) in the gut mucosa, ion transports, sulfation ability of xenobiotics, and many other things; sulfation ability is a key component in metabolising of drugs, for instance. Differences in our individual abilities to sulfate certain compounds give us at least one explanation as to why different people may respond differently two drugs treatment (see previous posts), and our ability to metabolise a common drug such as acetaminophen (paracetamol) can apparently be predicted form our preinterventional excretion of the microbial co-metabolite 4-cresyl sulfate; other gut microbial contributions that can alter the absorption, metabolism, and safety of drugs have been demonstrated recently.

Gastric bypass (Roux-en-Y) is a surgical procedure carried out to delay and reduce the absorption of calories and includes bypassing a large part of the stomach and a part of the small intestine by a procedure known as "stapling". Roux-en-Y appears to be associated with major and stable changes in the microbiota and in many microbially generated compounds, all of which are key components in the host-microbe metabolic axes. "This suggests that the microbiota is an essential part of the "gearbox" that connects the physical effects of bariatric surgery to the resulting beneficial effects."

Gut ecology changes with age, and current investigations seek to define the rationale of and potential for manipulating the microbiome of older people, for instance with pre- and probiotics, to secure higher microbiome diversity (high microbiome diversity appears to be beneficial) and resilience to antibiotics-induced changes in gut flora.

For those of you who nearly choked on "helminth therapy" - I may put up a post in the future on how helminths (and maybe other intestinal eukaryotes such as amoebae?) apparently play a role in the presentation and regulation of diseases such as asthma and inflammatory bowel diseases...

The cells of our intestinal microbiome outnumber our own body cells by 10 to 1. Within the next decade or so we will be able to extract a lot of information about how the bacteria and other "bugs" in our guts influence and contribute to health and disease. Importantly, we may have to realise now more than ever that "germs and bugs" and their actions and interactions can hold the key to a healthy life in ways that we wouldn't think were possible only a few years ago. This means that we should acknowledge that some bacteria and parasites may be a sign of a healthy intestinal environment / a healthy gut function, and that consumption of drugs such as antibiotics may produce shifts in our microbiota that may not necessarily be beneficial.


Iliev ID, Funari VA, Taylor KD, Nguyen Q, Reyes CN, Strom SP, Brown J, Becker CA, Fleshner PR, Dubinsky M, Rotter JI, Wang HL, McGovern DP, Brown GD, & Underhill DM (2012). Interactions between commensal fungi and the C-type lectin receptor Dectin-1 influence colitis. Science (New York, N.Y.), 336 (6086), 1314-7 PMID: 22674328
Holmes E, Kinross J, Gibson GR, Burcelin R, Jia W, Pettersson S, & Nicholson JK (2012). Therapeutic modulation of microbiota-host metabolic interactions. Science translational medicine, 4 (137) PMID: 22674556